Just a few comments from a non-expert:
CMU blocks, or portions thereof, can be used to support your rebar cage
off the ground. In my case, I pre-embedded pieces of angle iron in one
cell of a CMU block, and clamped my rebar cage to the angle iron. I
drove another section of angle iron through the other cell into the
ground, and then filled that cell with concrete to secure it in place.
See http://ab7e.com/AB7E_Tower_Page_4.html for details.
Paint on the rebar cage is a really bad idea and totally defeats the
purpose of a Ufer ground. You WANT the rebar to contact the concrete,
which itself is conductive when damp (which it will almost always be) so
that it distributes currents over a larger area and volume. Besides,
the alkalinity of concrete inhibits rusting ... you will almost never
see rebar totally encased in concrete have substantial rust even after
decades.
I can possibly understand the tower manufacturer not wanting the bolts
be the primary current path to the Ufer cage (although I'd bet those
bolts could handle a lot more current than the tower legs themselves),
but you definitely need to connect your tower to the rebar cage. In
most jurisdictions it would be a code violation not to.
Silver soldered connections for lighting protection is also a horrible
idea. The solder joint will have the highest volumetric resistance by
far and it will blow apart under a high current load. The best
connections are exothermic (Cadweld, etc), but a really good mechanical
connection is good if you can keep it from corroding. I could see
silver soldering a properly clamped mechanical connection to protect it,
but never rely on the solder to conduct the current.
If possible, ground the shields of your coax at BOTH the top and bottom
of your tower. Lighting is low frequency RF (low megaHertz region), and
a wire grounded at one end can still pick up considerable radiated
energy an appreciable portion of a wavelength further away.
Good luck with your project!
Dave, AB7E
On 4/18/2016 7:28 AM, Dick Blumenstein wrote:
Hello tower and antenna gurus-
I was hoping that some of you would critique my upcoming plans and
also answer any questions I have to see if they can be improved.
(Thank you in advance for spending the time wading through this
detailed description!!)
I'm a relatively new subscriber to this forum and definitely new to
owning a tower. I've read (a couple of times) what appears to be an
excellent book by Steve Morris (K7LXC) "Up The Tower" and am trying to
install this correctly the first time.
1. Tower - The tower is a 20 year old used 73' US Tower (Model
HDX-572) in fairly good condition. Before starting the installation,
I will pull it apart, replace the pulley sheaves and wire ropes as
well as wire brushing and spraying on a coat of high-zinc content cold
galvanizing paint. (It has a prop pitch motor for a rotator).
2. Concrete base - The recommended hole in the ground for the concrete
base is 5' x 5' x 8' deep. There are 6 @ 1" diameter A36 anchor bolts
required in the drawings which are specified at 27" long each. A major
bolt supplier wanted over $130 per bolt for them! Wow! After much
reading, it appears that I can possibly substitute an anchor rod (36"
long and threaded at each end) made to the ASTM F1554 Gr 55
specification that is also "S1 Weldable". I plan on putting a heavy
duty WELDABLE nut on the bottom of each rod and just tack weld the nut
in one place on the bottom to secure each nut. The anchor rod is hot
dipped galvanized all except for the bottom end which is threaded for
the weldable nut and then is cold spray galvanized afterwards. The
bolts are arranged in 3 pairs of two, so I plan on getting made 3@
1/4" thick A-36 steel plates (3" x 6") made to slip down over each
pair of anchor bolts and sit above each pair of welded on nuts. This
will properly space the bottom of each pair of "bolts" as well as
providing more pull-out strength to each pair of "bolts". At this
time, I see no other way of holding the anchor bolts in place while
pouring concrete except to suspend the tower plate as high as I can
over the hole so that I can have room to work and smooth out the
concrete below it when the hole is filled. Only 6" of threaded rod
sticks up above the top of the concrete, so it only gives me about 4"
of working height under the suspended base plate to work the top of
the concrete.
3. Rebar cage - There is a rebar cage surrounding the 6 anchor
bolts. Any suggestions on how to hold the cage off the bottom of the
hole and center it in the hole without having extra rebar hammered
into the sides of the hole to hold it into position? I would assume
that any rebar sticking out past the edges of the concrete into the
soil might provide a path for moisture to intrude into the concrete,
eventually corroding the steel. Also, should I spray some
anti-corrosive paint on the rebar cage? (See next paragraph concerning
a Ufer ground. I assume that a coating on the rebar cage would reduce
its grounding effectiveness.)
4. Tower grounding - I've read about "Ufer" grounds and then wrote to
US Tower to ask why the 6 anchor bolts were not connected in any way
to the rebar cage surrounding the bolts? They wrote back recommending
to NOT connect anything to the anchor bolts inside the concrete.
Hence, I plan to somehow connect all the steel rebar cage around the
anchor bolts to a copper strap and bring it to the surface of the
concrete pad to then run copper strapping to the base of each tower
leg. Each tower leg will then have a copper strap go out to some kind
of an array of multiple ground rods. Not sure how many ground rods I
should really put into an array off of each tower leg? I plan on
silver soldering all ground connections using silver solder and MAPP
gas. For instance, I may use a 2" wide solid copper strap from the
Ufer ground penetrating the concrete base, past and connected to each
tower leg and then on to the outlying ground rod array. Whenever the
copper strap encounters a ground rod, a hole is drilled in the strap
the size of the ground rod and silver soldered at the top of the
ground rod to the copper strapping. I will use a stainless
connector to connect each steel tower leg to the copper strapping.
(Any idea where to get them?)
5. In the ARRL ham radio book, it suggests to connect all the ground
rod system arrays that are around the tower with a large copper wire
loop and then take that ground line (at least 6 gauge or bigger) back
to the house where it connects up to the power company's ground at the
power entry into the house and also the antenna ground junction box
just outside my shack wall.
6. Antennas- I plan on having 3 antennas. The tower came with a
Hy-Gain LJ-205CA 20m 5-element yagi antenna that I will initially
install and then possibly replace with a 3 element SteppIR yagi in the
future. In the original tower setup there was a dipole with traps
that well served the previous owner, hung off the tower top that
covered 160 to 40m. I plan on keeping that. Furthermore, for local
communication, I plan to put above the yagi, a new 17' 2m/70 vertical
that also came with the tower. The previous owner did not use one,
but I would like to install an NN4ZZ "TiltPlate" so that when I crank
down and lower the antenna system to the ground I can maintain it
parallel to the ground without using ladders.
7. Antenna and rotator control wires arriving at the tower- There are
4 cables arriving at the tower; 3 antenna coaxes and 1 rotator control
cable. I plan to install a PVC box near the base of the tower into
which all the cables are fed. Inside will be a copper plate onto
which is attached a copper grounding strap from the grounding system
mentioned in #4 above. Each antenna coax will terminate in a
connector and go through a Polyphaser which is also grounded to the
strapping. The output of each Polyphaser will be a coax line which
then exits the PVC box and heads outside next to the tower to run up
the side of the tower to the antennas. In addition, inside the box,
will be a grounded Polyphaser IS-RCT Rotator Control Line Protector to
help protect the prop pitch motor and its controller back in the shack.
8. Controlling the "floppy" antenna and control cables? - In the ARRL
book, there was a picture of someone using a fishing net to let the
coax and control lines coil into it when the tower was cranked down.
Anyone have a better "looking" suggestion other than letting the wires
sit on the ground? (I plan on physically being at the tower each time
I raise or lower it.)
9. Antenna and rotator control wires leaving the shack- The antenna
wires are direct buried between the shack and the tower. Is it OK to
run the prop pitch controller cable right next to the antenna coaxes?
The distance from the shack to the tower might be about 40-50 feet. I
may possibly run them inside a drilled out pvc pipe for protection, or
just dig a trench and use sand as a filler around the direct burial
cables. Just outside of the wall of my shack will be another PVC box
slipped over a ground rod embedded in the soil. Inside the box will be
a copper plate assembly that is clamped onto the ground rod which also
holds 3 more Polyphasers. The antenna coaxes from the tower will go
through the Polyphasers before heading through the wall into my
shack. A 3" wide copper strap will go from that copper plate inside
the box, through the wall onto a large copper buss bar which serves as
a ground for all my shack equipment. Do I also need to put another
Polyphaser IS-RCT Rotator Control Line Protector, in this PVC box, too
(I suspect so)?
10. RF ground, too?- At my previous home, all I had were a bunch of
parallel vertical antennas suspended from a tree. I previously had 2
additional "RF grounds" (besides the regular ground rods) composed of
drilled out copper pipe, each located diametrically opposite each
other about 12 feet from the shack entrance ground rod and connected
to that ground rod. Each copper pipe was filled with salt to increase
its supposed effectiveness. Now that I am using a tower, do I still
need salted "RF ground rods" outside the shack wall in addition to the
regular ground rod?
11. Lighting rod protection - In Steve Morris' book "Up The Tower", he
suggests putting a lightning rod as the highest point above the
antenna system. However, I don't see how to do that. Won't that
effect the performance of the 2m/70 vertical right next to it?
Any suggestions would be appreciated from hams who have gone through
this before.
Thank you so much, and 73.
Dick, K0CAT
E-Mail: rcblumen@centurylink.net
//
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|